1. Field of the Invention
This invention pertains generally to exploration and production and, more particularly, to a method and apparatus for ascertaining a characteristic of a geological formation.
2. Description of the Related Art
The characteristics of geological formations is of constant interest in the exploration for and production of subsurface mineral deposits, such as oil and gas. Many characteristics, such as the hydrocarbon volume, porosity, lithology, and permeability of a formation, may be deduced from certain measurable quantities. Among these quantities are the non-invaded resistivity, flushed zone resistivity, and diameter of invasion in a formation. The quantities are typically measured by logging-while-drilling ("LWD") and wireline tools. The tool carries one or more sources that radiate energy into the formation and receivers that sense the result of the radiation. The detectors measure this result and either transmit the data back uphole or temporarily store it downhole. Typically, once uphole, the data is input to one or more formation evaluation models, which are software programs used to evaluate the geological formation from which the data was gathered.
Formation evaluation models classically assume thick beds within the formation that lie normal to the wellbore. These beds are also assumed to be homogeneous not only in composition, but in structure in all azimuths about the wellbore. Logging tools are designed and built with these assumptions as a guide. These assumptions simplify modeling the formations, which is valuable from the perspective of computing resources. However, they limit the accuracy of the results in circumstances where the formation's characteristics deviate from the assumptions.
Formation evaluation models typically give little regard to the side of the borehole tools measure or to whether they are azimuthally focused, because all directions are assumed to be the same. This is not a problem in thick beds with bedding normal to the wellbore, i.e., in situations where the formation structure actually matches the assumptions. But, as the bedding angle increases so that the bed is no longer normal to the wellbore, the information can become quite different from one side of the borehole to the other. Without processing, it is impossible to obtain accurate results when combining focused measurements (e.g., a wireline density measurement) and omni-directional measurements (e.g., a wireline induction resistivity measurement). The focused tool responds to one bed while the non-focused tool responds to the average of multi-beds. The geometrical effects of dip must be removed before meaningful processing can proceed.
Fluid distribution is another area that classical models ignore. In permeable, dipping formations, invasion of drilling fluid is often asymmetric because of gravity slumping of the filtrate. More rigorous two-dimensional interpretation models do include filtrate invasion, but ignore dipping beds and azimuthal variations of the invasion. Azimuthal variations are generally not of concern in vertical wells with bedding normal to the wellbore. However, they become important as beds begin to dip or the well becomes deviated. Such variations can be due to dip and asymmetric filtrate invasion.
Gravity also complicates an evaluation. It segregates invading filtrate from formation fluids if there is a density difference. This is especially pronounced in gas zones with large density contrast. Differential pressure between the mud column and the formation creates the initial invasion, normal to the wellbore. This invasion penetrates the formation only so far before gravity dominates at which point the majority of filtrate begins to flow downward rather than outward. "Down" does not have to mean toward the bottom of the hole; it could mean toward one of the sides of the hole, if that is the down direction of the bedding. The higher the vertical permeability the more obvious this effect. The heavier fluid will puddle at the first impermeable layer. This puddling can appear on wireline logs as an apparent water leg at the base of thick, highly permeable gas zones, even though those zones produced dry gas.
In vertical wells, thin, low permeability layers, which minimize segregation, often mask the effect. If the spacing between layers is less than the resolution of the logging tool, then they will not be detectable. In the case of dipping beds, the segregation effect is more obvious. All of the filtrate that leaves the well eventually migrates down dip, even the filtrate that leaves on the up-dip side of the wellbore. This increases the depth of invasion in one direction, making it more obvious on deeper reading logging tools and it creates azimuthal variations of fluids.
Thus, formation evaluations of deviated wells and wells with dipping beds are a challenge, especially with gas reservoirs. Log responses in these wells are often considered "unexplainable." Asymmetry, fluid distribution, and gravity contribute greatly to this problem because of the assumptions classical, one-dimensional and two-dimensional formation evaluation models embody. Even calibration of logs to core can be difficult because of the dramatic changes from level to level asymmetry can cause.
The present invention is directed to resolving one or all of the problems mentioned above.